The present invention relates to an automatic position control apparatus for a ground working vehicle equipped with a liftable working implement, and more particularly to such a control apparatus by which lifting and lowering of the working implement can be automatically controlled so that the deviation of the draft value of the working implement actually detected by a draft detector from the draft value preset by a draft setting unit is maintained within a permissible range preset by a sensitivity setting unit.
Automatic position control apparatus of this type are provided for maintaining the ground working implement at a constant level above the ground with a so-called position control system while, at the same time, preventing an engine stop of the working vehicle or damage to the implement with a draft control system responsive to the draft of the implement.
Such automatic position control apparatus heretofore known are adapted to automatically control the lift of the working implement during the entire period of time when the above-mentioned deviation is above the maximum permissible value of a preset permissible range. In this case, the automatic lift control is discontinued when the deviation has decreased to the permissible range, entailing electrical, mechanical or hydraulic hysteresis and a lag of operation. This leads to excessive lift control of the implement and excessive deviation of the implement from the specified level relative to the ground. Further when the permissible draft deviation range is narrow for high-sensitivity control, there arises another problem that the excessive lift control mentioned causes hunting which impairs stable control.
These problems are attributable to the fact that attention has been directed solely to the protection of the working vehicle and implement from excessive load of draft. In other words, it has not been contemplated to assure automatic high-sensitivity lift or lowering control with consideration given to the above-mentioned hysteresis.